The present invention relates to a containment vessel of a pressurized water reactor and to a nuclear power plant therewith.
Most light water reactors (LWRs) have a safety system such as an emergency core cooling system (ECCS). Reactors having an active component such as a pump are called “active safety reactors”. On the other hand, reactors with a safety system that has a passive component such as a tank are called “passive safety reactors”.
Known as a passive safety reactor representing boiling water reactors (BWRs) is the natural circulation cooling type passive safety BWR (ESBWR) (see, for example, IAEA-TECDOC-1391, “Status of advanced light water reactor design 2004,” IAEA, May 2004, pp. 207-231; the entire content of which is incorporated herein by reference). Known as a passive safety reactor representing pressurized water reactors (PWRs) is AP1000 (see, for example, IAEA-TECDOC-1391, “Status of advanced light water reactor design 2004,” IAEA, May 2004, pp. 279-306; the entire content of which is incorporated herein by reference).
In the ESBWR, the reactor core is contained in the reactor pressure vessel (RPV). The reactor pressure vessel is placed in the dry well (DW). The space above the RPV skirt and the vessel support of the dry well is referred to as “upper DW”, and the space below is referred to as “lower DW”. Below the upper dry well, a pressure suppression chamber (wet well: WW) is provided. The pressure suppression chamber contains suppression pool water (SP water) and gas phase above the SP water.
The dry well is connected to the suppression pool by twelve LOCA vent pipes. The dry well and the pressure suppression chamber constitute a primary containment vessel (PCV). A gravity-driven cooling system (GDCS) pool is provided in the upper DW.
If a loss-of-coolant accident (LOCA) causing coolant leakage occurs due to a rupture of a coolant pipe of the reactor or other reason, the pressure in the dry well would rise and push water level in LOCA vent pipes to a position of a horizontal vent. In this case, gas in the dry well would enter the suppression pool water. The suppression pool water condenses all steam in the gas, but noncondensable gas such as nitrogen cannot be condensed. The noncondensable gas inevitably flows into the gas phase of the pressure suppression chamber and will accumulate in the gas phase.
As this process proceeds, the dry well will be filled almost with steam. All noncondensable gases, such as nitrogen, that have existed in the dry well flow to the gas phase of the pressure suppression chamber. All driving energy in this process is the pressure of the steam released into the dry well.
The noncondensable gases are compressed in the gas phase of the pressure suppression chamber. As a result, the pressure rises in the pressure suppression chamber. Because this rise of pressure determines the final pressure of the PCV, the free volume ratio between the wet well and the dry well is desired to be more than about 0.6.
If the free volume ratio is as small as 0.1 for example, the pressure in the pressure suppression chamber will reach about 1 MPa (approx. 10 kg/cm2) even if only the compression of the noncondensable gases is taken into account. To maintain the free volume ratio at a large value, efforts have been made to reduce the free volume of the dry well as much as possible in designing the BWR.
In case the free volume of the dry well is very large, the free volume of the wet well should be also large accordingly. Thus, the containment may be irrationally designed as if it had two large dry wells practically. At a LOCA, dry well pressure is kept higher than wet well pressure as much as corresponds to the water head (level) difference between the LOCA vent pipe and the suppression pool. This pressure difference is about 0.05 MPa (0.5 kg/cm2) at most.
Above the dry well, a passive containment cooling system (PCCS) pool is arranged. The PCCS pool has a passive safety function, not using any active component, but utilizing gravity, pressure difference or natural circulation. The PCCS pool holds PCCS pool water. In the PCCS pool, a PCCS heat exchanger is arranged. The PCCS heat exchanger intakes the atmosphere in the dry well through a suction pipe and condenses the steam in the atmosphere. While the steam is being condensed, the noncondensable gases such as nitrogen contained in the atmosphere are guided into the suppression pool water by a PCCS vent pipe.
The condensed water is returned to the GDCS pool through a condensate water return pipe and introduced into the RPV again as ECCS water source. The driving force used when the PCCS intakes the atmosphere in the dry well and guides the noncondensable gases into the suppression pool water is the pressure difference (pressure gradient) maintained between the dry well and the wet well.
The submergence of the PCCS vent pipe in the suppression pool is set higher than that of the horizontal vent of the LOCA vent pipe. Hence, the LOCA vent pipes are no longer used for the condensation of steam, once the rapid blow-down just after the LOCA ends, and after a moderate and stable condensation of steam initiates, only the PCCS heat exchanger is used to condensate steam. Only the PCCS vent pipe is used to vent the noncondensable gases at this stage.
Thus, the PCCS has the function of venting the noncondensable gases into the pressure suppression chamber. Therefore, even if a severe accident occurs, and a large amount of hydrogen is generated, the PCCS is designed to prevent a loss of its passive cooling function due to stagnancy of the hydrogen in the PCCS heat exchanger. Without this function, although the PCCS heat exchanger can initially condense the steam efficiently, noncondensable gases such as hydrogen and nitrogen would be stagnant in the PCCS heat exchanger and it would immediately become unable to intake the steam.
On the contrary, as long as the pressure difference between the dry well and the pressure suppression chamber is kept, this pressure difference can be used as a passive driving force to intake and condense the steam in the dry well without limitation by venting the noncondensable gases at high efficiency. Therefore, if the PCCS heat exchanger and the PCCS pool water are designed to have an appropriate capacity respectively, the PCCS can be used for any water cooled reactor with any containment configuration and reactor thermal power. Namely, the PCCS feasibility depends on whether a pressure suppression chamber can be installed or not, in order to maintain a pressure difference between the nodes. And then, a pressure suppression chamber feasibility further depends on how large a free volume ratio between the wet well and the dry well can be.
FIG. 7 is a vertical cross sectional view of a containment vessel used in a conventional passive safety PWR (AP1000).
In AP1000, the reactor core 1 is contained in a reactor pressure vessel (RPV) 2. The reactor pressure vessel 2 is connected to two steam generators (SGs) 3 by both a cold leg pipe 4 and a hot leg pipe 5. A reactor coolant pump (RCP) 6 is directly attached to the bottom of the steam generator 3. These devices and pipes, which constitute a reactor pressure boundary, are all contained in a containment vessel (CV) 77.
The containment vessel 77 of AP 1000 is a most typical containment vessel, called “large dry CV”, for use in PWRs. The containment vessel 77 is made of steel, because it is designed to be cooled with the external air in case of an accident. Most PWR plant other than AP1000 rather use a large dry CV made of prestressed concrete.
In the containment vessel, an in-containment refueling water storage tank (IRWST) 8 is provided. The in-containment refueling water storage tank 8 works as a gravity-driven cooling system if a loss-of-coolant accident occurs due to a rupture of the cold leg pipe 4 or the like. This gravity-driven cooling system cooperates with other passive ECCS to fill the lower part of the containment vessel with water to a higher level than the cold leg pipe.
After that, it is designed that the recirc screen is opened, introducing the water always into the reactor pressure vessel 2 to cool the fuel in the reactor core safely. If the water introduced into the reactor pressure vessel 2 is heated by the decay heat of the fuel in the reactor core, steam is generated and the steam fills the gas phase of the containment vessel 77 resulting in a rise of the temperature and pressure in the containment vessel 77.
A shield building 71 is built outside the containment vessel 77. A cooling water pool 72 of a passive containment cooling system (PCS) is provided on the top of the shield building 71. The cooling water pool 72 is filled with PCS cooling water 73. In case of a LOCA, the PCS cooling water 73 drains onto the containment vessel 77. Air flows into the shield building 71 through a containment cooling air inlet 74 and then a natural circulation force raises the air through the gap between an air baffle 75 and the wall of the containment vessel 77 until the air is released outside through a containment cooling heated air discharge 76 formed at the top of the shield building 71. The drainage of the PCS cooling water 73 and the natural convection of air serve to cool the containment vessel 77 in safety.
In this way, AP1000 can cool the reactor core 1 and the containment vessel 77 with an extremely high reliability only by the passive safety systems requiring no external power source.
Although a rated electric output of AP1000 is about 1,117 MWe, the rated electric power can be easily increased up to about 1,700 MWe by increasing the number of steam generators to three. If the thermal output of the reactor core increases, however, the pressure in the containment vessel will rise at the event of a LOCA.
To mitigate the pressure rise in the containment vessel at a LOCA, the containment vessel may be made a little larger. Further, the containment vessel will become more reliable if a PCCS designed for passive safety BWRs is employed besides conventional functions of air and water cooling for the containment vessel cooling. The PCCS can attain cooling capability as high as necessary, merely by increasing the capacity of the heat exchanger and the amount of cooling water in accordance with the thermal output of the reactor.
In order to employ the PCCS, a pressure suppression chamber need to be provided so that a pressure difference between the nodes may be utilized as a passive driving force for venting noncondensable gases. The large dry CV of the PWR has a free volume as large as about tens of thousands of cubic meters. This volume is about ten times as large as the dry well free volume of the BWR. Therefore, if a pressure suppression chamber having the same volume as that of the BWR is provided, the pressure at an LOCA may reach about 1 MPa (approx. 10 kg/cm2) due to compression of the noncondensable gases to about one-tenth the initial volume of the gases. Consequently, the containment vessel (CV) may be ruptured.
That is, if a PWR tries to have a pressure suppression type containment and have a pressure suppression chamber like a BWR, the containment pressure will rather becomes extremely high. On the contrary, if the large dry CV is designed to withstand such a high pressure, the manufacturing cost will be practically too expensive.
The volume of the dry well of the BWR can be one tenth of the CV volume of the PWR, because the BWR has neither large device such as a steam generator nor large reactor coolant loops and has a few pressure boundary components that should be contained. On the contrary, in the PWR, the number of required steam generates increases in proportion to the thermal output. Therefore, the containment vessel of, for example, a recent four-loop PWR plant has a free volume of as much as about 80,000 m3. In such a large power four-loop PWR plant, passive cooling of the containment vessel can hardly be achieved.